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Based on kernel version 3.16. Page generated on 2014-08-06 21:36 EST.

1			Tagged virtual addresses in AArch64 Linux
2			=========================================
3	
4	Author: Will Deacon <will.deacon@arm.com>
5	Date  : 12 June 2013
6	
7	This document briefly describes the provision of tagged virtual
8	addresses in the AArch64 translation system and their potential uses
9	in AArch64 Linux.
10	
11	The kernel configures the translation tables so that translations made
12	via TTBR0 (i.e. userspace mappings) have the top byte (bits 63:56) of
13	the virtual address ignored by the translation hardware. This frees up
14	this byte for application use, with the following caveats:
15	
16		(1) The kernel requires that all user addresses passed to EL1
17		    are tagged with tag 0x00. This means that any syscall
18		    parameters containing user virtual addresses *must* have
19		    their top byte cleared before trapping to the kernel.
20	
21		(2) Non-zero tags are not preserved when delivering signals.
22		    This means that signal handlers in applications making use
23		    of tags cannot rely on the tag information for user virtual
24		    addresses being maintained for fields inside siginfo_t.
25		    One exception to this rule is for signals raised in response
26		    to watchpoint debug exceptions, where the tag information
27		    will be preserved.
28	
29		(3) Special care should be taken when using tagged pointers,
30		    since it is likely that C compilers will not hazard two
31		    virtual addresses differing only in the upper byte.
32	
33	The architecture prevents the use of a tagged PC, so the upper byte will
34	be set to a sign-extension of bit 55 on exception return.
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